Coulomb Explosion Dynamics of in Intense Laser-field: Identification of New Two-body and Three-body Fragmentation Pathways

-The Coulomb explosion process of N20 in an intense laser-field (5 PW/cm 2) has been investigated by the high-resolution time-of-flight (TOF) spectroscopy. Six two-body explosion pathways involving the NO +, NO 2§ N2 § molecular ions have been securely identified from the momentum-scaled TOF spectra of the fragment ions. Assuming a linear geometry, three-body explosion pathways were investigated by sequential and concerted explosion models. When the concerted model is adopted, the observed momentum distributions of six atomic ion channels; N +, N 2+, N 3§ O § 02+ and O 3§ were well fitted using the Gaussian momentum distribution with the optimized bond elongation factor of 2.2(3). From the yields of individual Coulomb explosion pathways determined by the fit, the abundance of the parent ions, N20 z§ (z = 2-8), prior to the twobody and three-body explosion processes was found to have a smooth distribution with a maximum at z 3. ~ T R O D U C T I O N The phenomenon called a Coulomb explosion o f molecules in an intense laser-field [1-18] is one o f the most fascinating research targets in recent photochemistry and photophysics, because the formation o f various singlyand multiply-charged fragments produced after the explosion with different kinetic energies carry a lot o f essential information regarding the interaction between molecules and a strong radiation-field. So far two different approaches have been adopted to investigate the Coulomb explosion in an intense laser-fields; one is the covariance mapping method developed by Frasinski and Codling [3 6], and Cornaggia and Normand [9-13], and 766 A. Hishikawa et al. the other is the mass-resolved momentum imaging (MRMI) method developed by our group [1,2]. The MRMI is the method based on the high-resolution measurements of the momentum-scaled time-of-flight (TOF) spectra of fragment ions, and we demonstrated that the high-resolution measurements of the released kinetic energies of the fragment ions is crucial to identify the Coulomb explosion pathways and to derive an angular distribution of certain fragment ions produced from individual pathways. Studies on the molecular Coulomb explosion in an intense laser-field were designed previously to know whether the multiple ionization of Xe atoms in an intense laser-field occurs through sequential steps or through collective or simultaneous emission of electrons [5]. By studying the Coulomb explosion of HI, which is isoelectronic with Xe, Codling et al. [ 19] observed that the released kinetic energy of the fragment protons was much lower than that expected from the Coulombic energy-release expected at the equilibrium internuclear distance of neutral HI, and concluded that the electron stripping proceeds sequentially, so that the internuclear distance of HI stretches at each ionization step. Recent experimental [1,2,7,8] and theoretical [16-18] studies on the Coulomb explosion process ofdiatomic molecules in an intense laser-field revealed that the formation of multiply charged parent ions is enhanced considerably at an elongated geometry in which the internuclear distance becomes approximately twice as large as an equilibrium internuclear distance re in the neutral ground state. Such a bond elongation process in the Coulomb explosion was also identified for some polyatomic molecules. The Coulomb explosion processes of C2H2 [9,10], CO 2 [11,13] and SO2 [1,12] showed that two equivalent chemical bonds of the three molecules tend to stretch to reach elongated bond distances, which are approximately twice as large as the corresponding equilibrium bond distances in the neutral electronic ground state. These results suggest that the Coulomb explosion of polyatomic molecule also proceeds through a similar mechanism as diatomic molecules. The bond elongation in diatomic molecules to reach the critical bond distance r c was previously ascribed to predissociative motion in the singly charged ion state [6,7] or dissociative recombination into the highly excited neutral states [14] of a parent molecule. In the case of an asymmetric triatomic molecule like N20, the charge distribution within a molecule just before the Coulomb explosion is expected to play a decisive role in determining the fate of the molecule whether it undergoes a fission of either of the two inequivalent chemical bonds or a simultaneous fission of these two bonds. Therefore, the information about the correlation between the charge stripping process and the nuclear motion occurring in a ultrashort time period would be derived from the identifications of the Coulomb explosion pathways of asymmetric triatomic molecules forming different charged Coulomb Explosion Dynamics of N20 in Intense Laser-Field 767 atomic and molecular ion fragments. The observation of the Coulomb explosion of N20 in an intense laser-field ( 1 PW/cm 2) was reported first by Frasinski et al. [4], who applied the three-dimensional covariance mapping technique to correlate the peaks in the TOF mass spectrum. They found that multiply charged N20 ions dissociate dominantly either through the two-body explosion pathway, N202+ ~-~ N + + NO +, or through the three-body pathway, NzO 6. --* N 2+ + N 2+ + O 2+. Since no feature assignable to the N2 O3+ ~ N + + N + + O + pathway was identified in their covariance map, it was inferred that six electrons are ejected all at once from neutral N20 for the three-body pathway. This implies that only N202+ and N2 O6+ were formed in the intense laser-field without producing the intermediate multiply-charged ions states, i.e., N=O s+, N2 O4+ and N205+. On the other hand, our recent investigation on N 2 [2] in an intense laser-field (0.73 6.9 PW/cm 2) exhibited a wide and smooth charge-number distribution of the parent ions. It would be worthwhile to clarify the origin of the propensity in the formation of the N206+ to understand the charge-stripping dynamics of polyatomic molecules in an intense laser-field. In the present study, we investigated the Coulomb explosion of N20 in an intense laser-field ( 5 PW/cm 2) using a high resolution (M/AM = 620) TOF spectroscopy. The high resolving power afforded us unambiguous assignments of fragment ion species with different kinetic energy distributions produced through the Coulomb explosion processes. From the identification of the fragment pathways, ultrashort nuclear dynamics associated with the multiple ionization and the Coulomb explosion is discussed. E X P E R I M E N T A L The details of our experimental setup have been presented previously [1,2]. Briefly, femtosecond laser pulses at 795 nm generated by a mode-locked Ti-sapphire laser (Spectra-Physics Tsunami) was introduced to a regenerative amplifier system (BM-Industry Alpha 10B/S) to obtain high-power low-repetition rate (10 Hz) short-pulsed laser light. After the pulse compressor, 100 fs laser pulses with the total energy as high as 50 m J/pulse were derived. The laser beam was focused by a quartz lens (f = 152 mm at 795 nm) onto a pulsed molecular beam between the extraction parallel repeller plates of a linear TOF mass spectrometer. The spacing between the repeller plates was measured carefully with a cylinder gauge to be 12.45 mm, which yields the extraction electric field of 233.7 V/cm. Typical mass resolution of m/Am 620 achieved under these experimental conditions enabled us to resolve the ion species produced after different multiply-charged states of parent molecules. The TOF mass spectra were 768 A. Hishikawa et al. recorded and averaged over 1 x 103 shots by a digital oscilloscope (LeCroy 9314) with a 400 MHz sampling rate. The N20 gas was introduced into the main chamber through a pulsed valve. When the pulsed valve was not operated, the background pressure in the main chamber was -3.3 x 10 -s Torr and that in the TOF tube was ~1.0 x 10 8 Torr. During the experiment, the pressure in the main chamber was kept sufficiently low (< 2 x 10 -7 Worr) in order to avoid the space charge effect. RESULTS AND DISCUSSION Figure 1 shows the TOF mass spectrum of N20 recorded at the laser-field intensity o f 5 PW/cm 2. Owing to the high resolving power of our TOF mass spectrometer, we were able to identify unambiguously a number of fragment ion species produced through the Coulomb explosion. The molecular fragment ions such as N22+ and NO z+ ions appear clearly in addition to NO +, which is the only diatomic ion species